Frequency modulation receiver



Aug. 28, 1945.

M. G. CROSBY FREQUENCY MODULATION RECEIVER Filed March 20, 1942 4 Sheets-Sheet 2 Ba/ancea modu/afor Z lzAmpl.

Saurce F) INVENTOR BY M rray G.Crosby A 6R EY Aug. 28, 1945- M. s. CROSBY FREQUENCY MODULATION RECEIVER Filed March 20, 1942 4 Sheets-Sheet 5 5 o ly/53F M 5/gna/ oufpuf 7b I. E Source Ba/ar/c/hq fube my. 8 lNVENTO Mufm 6. Crosby 4' ORNEY Aug. 28, 1945. M, CROSBY 2,383,847

FREQUENCY MODULATION RECEIVER Filed March 20, 1942 4 Sheets-Sheet 4 Mada/afar fube To Saurce Balancing fube l 1 l l l I L2 1.0 08 0.6 0.4-0.2 0 +0.2 0.4 0.6 0.8+].0 Modularinq voHage Fig. 10

INVENTOR Murray 6. Crosby ATTORNEY Patented Aug. 28, 1945 FREQUENCY MODULATION RECEIVER Murray G. Crosby,

Delaware Rlverhead, N. Y., asslgnor to Radio Corporation of America,

a corporation of Application March 20, 1942, Serial No. 435,467

9 Claims.

My present invention relates generally to angular velocity-modulated carrier wave receivers, and more particularly to a frequency modulation receiver wherein cancellation of amplitude modulation eflects is effected without a limiter and at a point following the balanced detector.

In the past, undesired amplitude modulation effects existing on an angular velocity-modulated carrier wave have been removed by means of a special limiter device, a balanced detector, or by a combination of both devices. By angular velocity-modulated carrier wave" is meant a phase, or frequency, modulated carrier wave. The expression timing-modulated may, also, be used hereinafter generically to designate such types of modulation. The limiter is effective in removing amplitude modulation at all times and even in the presence of frequency modulation, but the conventlonal balanced detector of frequency modulation signals is only completely balanced for the condition of no-frequency modulation present. Hence, when the balanced detector is depended upon to remove the undesired amplitude modulation the elimination is only complete in the absence of the desired frequency modulation, and

becomes progressively poorer as the desired modulation is increased.

In my application Serial No. 416,443, filed October 25, 1941, granted November 28, 1944 as U. S. Patent No. 2,363,649, there has been disclosed the utilization of detected amplitude modulation existing on the angular velocity-modulated carrier wave to modulate the detected modulated carrier wave energy with such polarity that the undesired amplitude modulation is greatly reduced. Hence, if desired, the special limiter stage may be dispensed with, since the elimination of the amplitude modulation by the balanced detector is made more complete. By omitting the limiter the gain of the receiver may be decreased, since the large signal voltages required to saturate the limiter do not have to be provided. Economy of construction and simplification follow as a result of gain reduction prior to the demodulator.

One of the main objects of my present disclosure is to provide an improved type of inverse modulation circuit for eliminating undesired amplitude modulation in frequency modulation reception; by properly adjusting the operational characteristic of the inverse modulator tubes, a more complete inverse modulation may be effected so as more completely to remove the amplitude modulation.

Another important object of my invention is to output are compensated for.

Yet another important object of my invention is to provide a frequency modulation detector circuit using a compensating modulating process at a point subsequent to the detector for eliminating amplitude modulation effects, and the modulation characteristic being given a non-linear form thereby to secure a more complete elimination of the said amplitude effects.

Still other objects are to provide improved and efficient detectors of frequency and phase modulated carrier waves which need no limiter stage ahead of them, and which detectors are simple to construct and are reliable in operation.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best, be understood by reference to the following description taken in connection with the drawings in which I have indicated diagrammatically several circuit organiza.v tions whereby my invention may be carried into effect.

In the drawings- Fig. 1 is a schematic representation of the I invention,

Fig. 1A shows a simple form of modulator, Fig. 2 illustrates one form of the invention, Fig. 3 is a modification of the system of Fig. 2, Fig. 4 is another modification,

Fig. 5 shows still another embodiment,

Fig. 6 shows a modification of the arrangement in Fig. 3 wherein the modulator tubes use special modulating grids,

Fig. '7 illustrates a modification of the embodiment in Fig. 6,

Fig. 8 shows a different form lator,

Fig. 9 shows a modification of the system of Fig. 8, and a preferred embodiment,

Fig. 10 graphically illustrates the various modulation characteristics which can be employed.

Referring to the accompanying drawings, wherein like reference characters in the various figures denote similar elements, the basic circuit of the inverse modulation system is shown in Fig. 1. This is the type of system shown in my aforeof balanced modusaid patent.- The I. F. (intermediate frequency) channel of the FM (frequency modulation) receiver feeds a balanced frequency modulation detector which is capable of producing the detected frequency modulation and the detected amplitude modulation (AM). The detected frequency modulation signal is then fed to a balanced modulator in which the amplitude of this detected output is modulated (in proper inverse phase) by the detected amplitude modulation signal output. In this way as the undesired amplitude modulation swings the amplitude of the signal, and in turn the amplitude of the detected FM output, upward, the amplitude modulation detected output which is fed to the balanced modulator has such a polarity as to decrease the gain and thereby cancel the upward swing of the detected FM output. The downward swings of modulation are similarly cancelled. This process is more completely considered in my aforesaid patent, and particularly in Figs. 1A and 1B thereof.

Fig. 1A shows the conventional circuit of a balanced modulator as described by Carson. The usual and well known function of this type of modulator is to produce an amplitude modulation with the elimination of the carrier. However, its function in this inverse modulator circuit is to provide a modulation of the detected FM signal, and to balance out the modulating voltage consisting of the detected AM signal. For this arrangement, the detected AM signal is fed cophasally to the modulator grids so as to balance out in the push-pull output transformer C. The operation is somewhat in the manner of a push-pull amplifier, having input and output transformers A and C, which has its gain modulated in accordance with the detected AM signal applied at common grid transformer B. Since the modulating AM signal is fed in push-push, it is balanced out in the output C.

Fig. 2 shows how the balanced modulator of Fig. 1A is connected to a frequency modulation detector of the type employing the split-resistor connection of my application Serial No. 402,784, filed July 17, 1941, granted September 15, 1942, as U. S. Patent No. 2,296,092. In Fig. 2 there is shown only so much of an FM receiver as is essential to a proper understanding of the invention. Let it be assumed that the balanced detectors 4-5 are preceded by the usual pre-demodulator stages of a superheterodyne receiver adapted to operate in the ultra-high frequency band, say 42 to 50 megacycles (mo), in which FM reception is had. As is well known, the received FM wave is produced by deviating the carrier frequency at the transmitter in accordance with the modulation amplitude, the rate oi. deviation being a function of the modulation frequencies per so.

A phase modulated wave (PM) may be considered as an FM wave which has been modulated with an input proportional to the modulating frequency. At present, a maximum carrier, or center. frequency (Fe) deviation of 75 kilocycles (kc) is used for FM broadcasting. In the superheterodyne receiver the FM waves are reduced to an intermediate frequency (I. F), although the same frequency deviation range is retained. The I. F. value may be chosen from a range of approximately 4 to 20 me. The FM waves, with center frequency of the I. F. value, are applied to the input electrodes of the I. F. amplifier. This invention is applicable to FM waves having a high or low frequency deviation range.

Diodes l and I are coupled to the series output circuits I and 3 of amplifier I by a network which has been disclosed in my aforesaid U. 8. Patent No. 2,296,092. Such a detection network provides balanced detected FM voltage and a separate AM voltage. The anode of diode 4 is connected to its cathode through a series path comprising secondary coil S, resistor R1 and resistor R2. The two resistors are shunted by the by-pass condenser C1. The anode of diode 5 is connected to its cathode through a series path comprising the secondary coil 81, resistor R: and resistor R4. The by-pass, condenser C2 shunts R3-R4.

Each of tuned circuits 2 and 3 is magnetically coupled to its respective secondary coil 8 and S1. Circuits 2 and 3 are equally and oppositely mistuned relative to the operating I. F. value by any predetermined frequency separation. This provides FM discriminator action. Detected FM voltage is developed across resistors R2 and R4 which are arranged in polarity opposition, it

' being noted that the cathode end of resistor R4 is at ground potential.

The detected AM voltage may be taken oi! at the anode end of resistor R1. The well known type of discriminator shown herein converts applied FM waves to AM waves with modulation envelopes 180 degrees out of phase. These modulation envelopes are rectified by diodes 4 and 5 which make the rectified outputs available across resistors R1R2 and R:- The manner in which RIR2 and Rs-Ra are connected together makes the detected FM voltage available across H's-R4. The detected AM voltage is taken from across resistors R; and R4. It will be noted that the two resistors R1 and R4 are connected so that they both have their ends which are towards the diode at high potential, and the combination of the detected AM voltage outputs is in phase. The normal adjustments of the resistors are to make them all equal in order to effect balanced conditions for both AM and FM reception. Bypass condensers will by-pass the I. F. currents fed to the discriminator, but allow the modulation frequencies to pass. There will, then, be derived from the upper end of R: the detected FM signal voltage, and the latter has superposed thereon the detected AM signal voltage to be eliminated. The detector AM signal votage is derived from the anode end of R1.

The detected FM signal voltage is applied to the modulator tubes 8 and 9 in push-pull relation. Push-pull transformer 6 has its primary ends connected to the cathode ends of Rr-Rq. The detected amplitude modulation voltage is applied to tubes 8 and 9 by the common-leg transformer I. Isolating resistors R and R. prevent the respective transformers 8 and I from shunting the diode resistors. Isolating, or blocking, condensers may be substituted for resistors R and R. The output transformer il) balances out the amplitude modulation component which is amplified by the modulator tubes, and delivers the FM component which has been modulated so as to remove its undesired amplitude varia tions.

Fig. 3 shows another arrangement of the type of circuit of Fig. 2. It will be noted that the transformer l for feeding the AM component to the common leg of the modulator input circuit has been eliminated, and the common-leg circuit is fed directly from the diode resistors. This is possible since with the slip-resistor type of diode connection, a point is available which gives the sum of the diode outputs for the amplitude modulation as well as the difference output for frequency modulation. In the circuit of Fig. 3 the tubes 8 and 9 are biased by biasing resistors II, each bypassed for audio frequency currents. The point C of resistor R1 is connected by condenser 8' and lead Hi to the midpoint of the secondary of transformer 6. Resistor 9' is connected to ground from lead Ill to provide a direct current return path for the input grids of tubes 8 and 9.

Study of the circuits of Figs. 2 and 3 will reveal that the voltage applied to the modulator grids is in each case the summation of the detected FM and AM components. Such a summation of voltages is present in each diode resistor taken separately. Hence, the circuit of Fig. 4 is the electrical equivalent of the circuits of Fig. 2 and 3. That this equivalence is true may be seen from the following analysis.

Referring to Fig. 3, the voltages fed to the two modulator grids are E1 and En. E1 is equal to the summation E3+E4 an E: is equal to the summation E's-Ea. The voltage E3 is equal to the summation of the voltages across A and AC. The

voltages E4 and E5 (assuming a one-to-one transfer through transformer B and resistance R) are show that the voltage E2 is equal to 2(AB). Hence, if the separate diode voltages from each diode are fed directly to the modulator grids, the same voltage distribution will be obtained as is obtained in Fig. 3, or in Fig. 2.

In the circuit of Fig. 4, the bias for the modulator grids of tubes 8 and 9 is obtained by connecting the respective grids directly to the anode side of the diode resistors 4' and 5'. The anode side of each of the diode resistors must be connected to the grids so as to produce a negative bias which increases when the modulated carrier voltage increases. The common junction of resistors I and 5', each properly bypassed for I. F., is grounded, and each diode cathode is grounded. With this connection an increase in carrier strength, as would be caused by an upward swing' of amplitude modulation, produces an increase in negative bias to the grids of the two modulator tubes 8 and 9. This reduces the gain of the modulator tubes so as to compensate for the increase in FM output caused by the increase in carrier strength. The opposite change in gain occurs when the carrier amplitude decreases. The resultant effect is a smoothing out of the amplitude variations of the FM detected output. The PM output of the detectors appears as a push-pull voltage on the diode resistors, and is combined in push-pull output transformer iii. The amplitude variations appear cophasally, and are therefore balanced out by the push-pull transformer Ill.

The circuit of Fig. 4 may be connected so as to prevent the modulator bias from being varied by the carrier variations by using the circuit of Fig. 5. Blocking condensers l3 and I3 and respective grid-return resistors i2 and I2 are inserted so as to eliminate the direct biasing connection to the diode resistors 4 and 5'. This blocks the permanent direct current component, and allows the audio variations to pass. Proper bias is applied resistor R to the cathode and of resistor R2.

to the modulator grids of tubes 8 and 8 by means of the conventional bypassed cathode resistors l4 and ll. The anode and of resistor 4' is coupled to the grid of tube 8 by condenser I2, and condenser l3 couples the anode end of resistor 5' to the grid of tube a. i

In the above-described circuits the type of modulation use in the balanced modulator is grid modulation. This type of modulation has various disadvantages with respect to the auxiliary-grid type of modulators about to be described. With grid modulation the operational characteristic must be adjusted non-linear to effect modulation. Consequently, the characteristics of the modulator tubes as amplifiers of the detected FM component are non-linear, and distortion is encountered. The even harmonics of this distortion are cancelled by the balancing action of the push-pull circuits. The distortion may also be reduced by designing the slopes of the input discriminator network 2-3 to be gradual. As a consequence the detected FM component will be small, and will swing the modulator tubes over a small range of their characteristics thereby to produce low distortion. This feature of utilizing low-slope discriminator curves in the detection circuit is a feature of the invention as embodied in Figs. 2 to 5.

In the auxiliary-grid type of modulator circuit of Fig. 6 the functions of amplification of the FM component, and modulation by the AM component, are separated. Hence, each may be adlusted to be best suited to its function. As will be shown later, in order to effect a more complete degree of inverse modulation, the modulation characteristic must be adjusted so as to cause distortlon if the same characteristic is used for amplification. Hence, a separation of these two functions prevents introduction of distortion to the FM component.

Fig. 6 is the same as Fig. 3 in all respects, except that the detected amplitude modulation component is fed to a separate modulating grid of each modulator tube. Hence, the tubes 20 and 2| may be adjusted so that the grids 24 and 25, which are fed the FM component by transformer ii, are operated on the linear portion of their characteristics, while the modulator grids 22 and 23 may be adjusted with high bias so as to produce the proper modulation characteristic to be described later.

Specifically, the primary winding of transformer 6 has its ungrounded end connected by The tubes 20 and 2!, each of which may be of the pentagrid type BSA? for example, have the cathodes thereof connected by biasing resistors 26 and 26'. Each biasing resistor is properly bypassed for audio currents, and the junction of the resistors is connected to the grounded midpoint of the secondary of transformer 6'. Auxiliary grids 22-23 are connected in common, through coupling condenser l3", to the anode end of R1. Resistor i2" provides a grid return path for the grids 22-23. The grids 24 and 25, each located in a positive shielding field, are connected to opposite ends of the secondary of transformer 6'.

Fig. '1 shows another modification of the auxiliary-grid type of modulator. In this circuit, the midpoint of the diode series resistors 3lI3I is grounded, and a common resistor 29 is inserted to obtain the detected AM component. Resistors ill and 31 furnish detected FM signals in push-pull to the inner grids 22 and 23 of the modulators. The detected AM component will, also, be present on these resistors, but since it appears cophasally transformer It will balance it out in the output circuit. The inner grids 22 and 23 are adjusted to be linear so that there is no intended modulation taking place on these grids. The outer grids 24 and 25 are inversely modulated by the'detected AM voltage of the common resistor 25 in the diode circuits.

Considering the circuit of Fig. 7 in detail, the I. F.-tuned output circuit 2" 01' the prior amplifier is magnetically coupled to each of input circuits 2' and 3. The circuits 2' and 3' are oppositely and equally mistuned by equal frequency values to provide a proper discriminator action. This has been explained in connection with Fig. 2. Common resistor 29, properly bypassed fo I. F. currents, is connected between the junction of 3B3i and the junction of circuits 2'-3'. The cathode end of resistor 30 is connected to grid 22 by coupling condenser 34, grid return resistor 32 connecting the grid to the grounded junction oi. the usual biasing resistors. Similarly, condenser 85 couples the cathode end of resistor 3| to grid 23, grid return resistor 33 connecting to ground.

The outer grid 24 is coupled by condenser 38' to the anode end of common resistor 29. The outer grid 26 is connected by condenser 38 to the anode end of resistor 29. Resistors 31 and 31 act as grid return paths for grids 25 and 24 respectively.

In the type circuit shown in Fig. 8, the efiect of a balanced modulator is obtained by using a single modulator tube and a balancing tube. Th balancing tube is arranged so that it causes a cancellation of the undesired detected AM component which is amplified by the modulator. This circuit is a modification oi the type of circuit disclosed in my aforesaid U. 8. Patent No. 2,363,649. Tube 20 is the modulator tube which is 01' the two-grid type with the detected FM component fed from potentiometer 38 to the outer grid 24, and the detected AM component fed to the inner grid 22 by means of potentiometer 39. The detected AM component is fed to the inner grid 23' of a balancing tube 2! by means of a potentiometer 40. Tube 2| is shown of the negative transconductance type which is described in my application Serial No. 420,500, filed November 26, 1941. By leaving the screen resistor 44 unbypassed the phase of the amplified voltage is reversed with respect to what it would be if the screen did not contain an impedance. Thus, the detected AM component amplified by the modulator tube has the usual phase reversal effected by an amplifier tube, but the balancing tube 2! does not. Consequently, equal voltage's fed to their two inner grids 22 and 23' will balance in the common output circuit consisting of resistor The plates of tubes 20 and 2| are connected in parallel to the upper end of load resistor 46. The screen grid of tube 20 and that of tube 2| are connected to the positive potential supply by series resistors 43 and 44, the resistor 43 being bypassed by condenser 45 to ground. Potentiometer 38 has its ungrounded end coupled to the cathode end of R: by condenser 38'. The adjustable tap of potentiometer 38 is connected to outer grid 24 of modulator tube 20. The ungrounded ends of potentiometers 39 and 40 are connected in common, and coupling condenser 39' connects the common ends to the anode end of R1. The outer grid 25 of balancing tube 2i is grounded, while the inner grid 23' isconnected to the adjustable tap of potentiomete 40. The inner grid 22 of modulator tube 20 is connected to the adjustable tap at potentiometer 39. The usual bypassed biasi'ng resistors, in the cathode circuits of tubes 20 and 2 I provide negative grid bias for the inner and outer grids of each of tubes 20 and 2|.

Fig. 9 shows a further modification of Fig. 8. The same principle of using a modulator and a balancing tube is used, but instead of using a negative transconductance tube with a common output resistor, duplicate tubes 20 and 20' are used in conjunction with a push-pull output transformer ill. The detected AM component is fed directly to the inner grid 22 of the modulator tube 20, and through potentiometer 50 to the inner grid 23" of the balancing tube 20'. Hence, the detected AM component may be balanced out by varying potentiometer I50.

Considering the circuit of Fig. 9 more specifically, it will be observed to be quite similar to that shown in Fig. 8, except for the differences which have been described. The potentiometer 38 is connected at its upper end, and-through coupling condenser 38, to the cathode end of resistor R2. The lower end of the potentiometer is connected to the grounded and of resistor R4. The outer grid 24 01' modulator tube 20 is connected to the adjustable tap of potentiometer 38. The outer grid 25" of balancing tube 20' is connected to the grounded Junction of the cathode biasing resistors of each of tubes 20 and 2B. The ungrounded end of potentiometer 5B is coupled by condenser ii to the anode end of resistor Rl. The adjustable tap of potentiometer 50 is connected to the inner grid 23" of balancin tube 20'.

In my aforesaid U. S. Patent No. 2,363,649, I described the modulator tubes of this inverse modulator system as being linear modulators. The theory developed in that application showed that this type of modulator was capable of reducing the undesired modulation, but that there was a residual amount left which consisted mostly of second harmonic of the undesired component. In the improvement about to be described it will be shown that by adjusting the modulators to have a certain type of nonlinearity, the degree of removal of the undesired AM component will be considerably increased.

The reason that the linear type of modulator cannot remove all of the undesired amplitude modulation may be seen when it is realized that as the amplitude modulation swings the amplitude of the signal down towards zero, the compensating inverse modulation must swing the gain of the modulator towards infinity. Obviously infinite gain is not realizable, so that percent amplitude modulation may not be removed. However, limiters and other similar devices are, also, not capable of removing 100 percent modulation. On the other hand, for degrees of modulation that are less than 100 percent, an approximation of this ideal modulator characteristic with a gain rising towards infinity may be realized. At any rate, an improvement over the linear modulator is possible.

Let it be assumed that an incoming frequency modulated carrier is modulated 50 percent by the undesired amplitude modulation. The output of the balanced detector will then be the desired detected FM signals with a 50 percent amplitude modulation envelope superimposed upon it. Such a wave is shown as Fig. 1A in my aforesaid U. S. Patent No. 2,363,649. Let it, also, be assumed that the amplitude of the detected FM output is equal to unity, so that the amplitude modulation swings it up to a peak amplitude of 1.5 and down to an amplitude of 0.5. In order for the inverse modulator to remove this amplitude envelope, it

I 9,883,8i7 must have a gain at 2.0 at the instant when the must be 1/135, 010.8. and at the instant it 18 at 0.75. the gain must be 1.333. In this way the ideal modulation characteristic may be plotted as shown by the solid line curve in Fig. 10. Modulating voltage" is plotted as the horizontal axis.

For 100 percent amplitude modulation, th peaks of this modulating voltage would be plus and minus 1.0. "Gain modulator" is plotted on the vertical axis. The curve would rise towards infinity at the 100 percent modulation peak of +1.0. It would drop to 0.5 at the peak of -i.0. Only the range for 50 percent modulation is plotted. The curve for linear modulation is shown by the dashed line. It can be seen that the ideal modulation characteristic departs considerably from the linear characteristic. The dotted line curve shows how the ideal characteristic may be approximated by the use of a. type BSKT tube it is only required to secure separate detected FM and AM components from the detector circult regardless oi. the construction thereof.

While I have indicated and described several systems (or carrying my invention ,-into eil'ect, it will be app -rent to-one skilled in the art that my invention isby no means limited to the particular-organizations shown and described. but that many modifications may be made without departing from the scope oi my invention, as set forth in the appended claims. What I claim is:

i. In a frequency modulated carrier wave system, a detector for such waves constructed to biased at -15 volts. The tube would be used as 'a grid modulator to obtain this characteristic.

6.' 7. 8, and 9 may be adjusted to have the desired modulation characteristic of the solid line of Fig. 10. This can be done by adjusting the bias on the grid to which the detected amplitude modulation component is applied. Proper choice of other element voltages, as well as choice of tubes. will also be required. In general, bias which places the operating point near negative cut-oil will give the desired curvature to the characteristic.

An interesting, and highly important. eiIect observed in an actual experimental embodiment of the circuit oi Fig. 9 was the removal of distortion caused by the selectivity of the I. F. ampliiier preceding the discriminator, Such an amplifier has a round-topped response curve, and

of necessity introduces amplitude modulation into the FM wave. of course, it would be desirable to have the I. F. amplifier provided with aiiat top response curve. but this is diflicult to secure by the usual and known methods. In the absence of the presently disclosed inverse modulation, or of a special limiter stage, distortion as high as 12% is found introduced by the usual I. F. amplifier response curve. The distortion. when employing the present invention, or a special limiter stage, can be reduced to 2.5%.. In other words. the present invention is capable of equaling the performance of a special limiter stage in respect oi compensating for AM eiIects introduced by virtue of the round-topped selectivity of the I. F. amplifier.

produce a first modulation .voltage corresponding to the frequency modulation component or the modulated waves and a second voltage correspending to an amplitude modulation component of said waves, means [or inversely modulatlns said iirst modulation voltage with said second voltage. said last means comprising a first tube having at least two'spaced control elements and an output electrode, means tor applylns' said two voltages separately to respective ones of said two control elements, a second tube having at least two spaced control elements and an output electrode, means applying at least said second voltage to one o! the second tube control elements, means for biasing at least said one control element of said second tube sufllciently negative to place the tube operating point near negative cutoil thereby to provide a desired curvature in the tube operating characteristic, and means connecting the output electrodes of both tubes in a sense to balance out said second voltage.

2. In a irequency modulated carrier wave system, a detector for such waves constructed to produce a first modulation voltage correspondin to the frequency modulation component or the modulated waves and a'second voltage corresponding to an amplitude modulation component 0! said waves, means tor inversely modulating said first modulation voltage with said second vo iasc. said last means comprisin a first tube having at least two spaced control elements and an output electrode, means to: apply ls said two voltages separately to respective ones of said two control elements. a second tube having at least two spaced control elements and an output electrode, means applying said two voltases separately to respective ones oi the second tube control elements, means tor biasing close to cut-oil the respective control elements oi said two tubes to which said second voltage is applied. and means connecting the output electrodes oi both tubes in a sense to balance out said second voltage.

8. In combination with a transmission circuit tor angular velocity-modulated carrier waves. said inator the diodes are coupled in opposition to the secondary circuit or the discriminator, and both primary and secondary circuits are tuned to the same carrier irequency value. In general,

circuit having a round-topped selectivity response characteristic thereby introducing an amplitude modulation effect on said waves, means detecting the transmitted waves. means to provide irom the detected waves a voltage component corresponding to the angular modulation and a second voltage corresponding to the amplitude modulation, at least two electron discharge devices, one device having at least two spaced input electrodes and an output electrode, the second device having at least one input electrode and an output electrode, means to apply said two voltages separately to said spaced electrodes at said one d1. vice, means to apply said second voltage to the input electrode or said second device, means ior biasing said second device input electrode sunlciently negative to impart a desired degree oi non-linearity to the operating characteristic of the second device, and means for connecting the output electrodes of said two devices in a sense to balance out the effect of said amplitude modulation voltage.

4. In a timing modulated carrier wave ystem, a detector for such waves constructed to produce a first modulation voltage corresponding to the timing modulation component of the modulated waves and a second voltage corresponding to an amplitude modulation component of said waves, a first tube having at least two spaced control elements, means for applying said two voltages separately to respective ones of said two control elements, a second tube having at least two spaced control elements, each tube having a respective output electrode, means applying at least said second voltage to one of the second tube control elements, means for negatively biasing each control element of said two tubes to which said second voltage is applied close to cut-oll, and means connectin the output electrodes of both tubes in a sense to balance out said second voltage.

5. In a frequency modulated carrier wave system, means to derive from such waves a first modulation voltage corresponding to the frequency modulation component of the modulated waves and a second voltage corresponding to an amplitude modulation component of said waves, a first tube having at least two spaced control grids, means for applying said two voltages separately to respective ones of said two control grids, a second tube having at least two spaced control grids, means applying at least said second voltage to one of the second tube control grids, each tube havin a respective output electrode, means for applying a relatively high negative bias to said one control grid of the second tube, and means connecting the output electrodes of both tubes in a sense to balance out said second voltage.

6. In combination with a transmission circuit for angular velocity-modulated carrier waves, said circuit having a round-topped selectivity response characteristic thereby introducing an amplitude modulation eflect on said waves, means to derive from the waves a voltage component corresponding to the-angular modulation and a second voltage corresponding to the amplitude modulation, at least two electron discharge devices, one device having at least two spaced input electrodes and an output electrode, the second device having at least one input electrode and an output electrode, means to apply said two voltages separately to said spaced electrodes of said one device, means to apply said second voltage to the input electrode of said second device, biasing means connected to the input electrode of said second device for applying a high negative bias thereto, and means for connecting the output electrodes of said two devices in a sense to balance out the eiIect of said amplitude modulation voltage.

7. In combination with a transmission circuit for angular velocity-modulated carrier waves, said circuit having a round-topped selectivity response characteristic thereby introducing an amplitude modulation eflect on said waves, means to derive from the waves a voltage component corresponding to the angular modulation and a second voltage corresponding to the amplitude modulation, at least two electron discharge devices, one device having at least two spaced input electrodesand an output electrode, the second device having at least one input electrode and an output electrode, means to apply said two voltages separately to said spaced electrodes of said one device, means to apply said second voltage to the input electrode of said second device, means for biasing the latter input electrode near cut-oil, and means for connecting the output electrodes of said two devices in a sense to balance out the eilect or said amplitude modulation voltage.

8. In a system for receiving timing modulated carrier waves, a detection circuit having an input network upon which is impressed said waves, means in said circuit for deriving from the detected waves a modulation voltage corresponding to the timing modulation and asecond voltage corresponding to an undesired amplitude modulation, a pair of electron discharge tubes, each tube having a pair of control electrodes and an utput electrode, means applying the timing m0dulation voltage to like control electrodes of said tubes. meansapplying the second voltage to the second pair of like control electrodes, means connecting the output electrodes of said tubes in push-pull relation thereby to cancel out said undesired modulation, means biasing said first like electrodes whereby they are operated at the linear portions of their characteristics, and means blasing the second pair of like control electrodes highly negative.

9. In a frequency modulated carrier wave system, means to derive from such waves a first modulation voltage corresponding to the frequency modulation component of the modulated waves and a second voltage corresponding to an amplitude modulation component of said waves, a first tube having at least two spaced control grids, means for applying said two voltages separately to respective ones of said two control grids, a

second tube having at least two spaced control grids. means applying at least said second voltage to one o! the second tube control grids, and means to apply a high nemative bias to the grid of the first tube and the said one grid of the second tube to which said second voltage is applied, each of said tubes having a respective output electrode, means connecting the output electrodes of both tubes in a sense to balance out said second voltage.

MURRAY G. CROSBY. 

